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Tag Archives: Zimmer

Fracture of Highly Crosslinked UHMWPE Acetabular Liners

Four well-placed crosslinked UHMPWE acetabular liners of distinct design were retrieved subsequent to fracture failure of the rim. In each case, electron microscopy of the fracture surface revealed fatigue initiation markings associated with a stress concentrating feature in the rim. Fracture surface investigation demonstrated that crack propagation in all cases followed contours of tensile stresses. FEA simulations were performed to examine whether direct rim loading could be the cause of the observed failures.

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Posted in Hip| Tagged , , |

High Flexion in Contemporary Total Knee Design: A Precursor of UHMWPE Damage? A Finite Element Study

The success of total knee arthroplasty has contributed to its widening application to a younger, more active patient population who daily regimen includes more demanding high flexion activities. Worldwide expansion to Middle Eastern and Asian populations, where the attainment of high knee joint flexion is often a cultural requirement, has been steadily increasing in recent years. This study reveals the contact areas and stresses that are associated with polymer insert abrasion in four total knee designs during the most highly loaded portions of three different high flexion activities, and suggests their efficacy in clinical use.

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Posted in Knee| Tagged , , , |

Biomechanical Properties of Fixed-Angle Volar Distal Radius Plates Under Dynamic Loading

Distal radius fractures are commonly encountered in general orthopaedic and hand subspecialty practices. Most surgeons are comfortable with both operative and nonoperative management of these fractures. Treatment options have evolved with fracture pattern governing the specific treatment modality. Casting with or without reduction, percutaneous pinning, external fixation, and open reduction with internal fixation employing dorsal, volar and fragment specific plates are all common methods used to treat these injuries. A paradigm shift has occurred in the treatment of dorsally displaced distal radius fractures. Previous volar plating techniques demonstrated a high failure rate when compared to distal buttress plating which prevented fracture settling and recurrent displacement. Orbay and others have developed volar plating constructs, which provide subchondral support to the distal radius, transferring radiocarpal forces experienced in the postoperative period to the plate and volar cortex. Previous studies have examined biomechanical differences between dorsal and volar plating while further investigations between specific volar plate constructs under static and dynamic loading conditions have been reported. This study compares the biomechanical properties of eight different fixed-angle volar distal radius plate designs under dynamic loading to determine their ability to withstand the forces which occur during fracture healing and early postoperative rehabilitation.

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Highly Crosslinked Polyethylenes: Hopes vs. Realities

The enduring success of the low-friction arthroplasty first advanced by Sir John Charnley as a solution for severe hip arthritic problems may be appreciated from the fact that in 2003 more than 800,000 hip and knee arthroplasties were performed in the United States. The prevalence of aseptic loosening attributed to ultra-high molecular weight polyethylene (UHMWPE) debris-induced osteolysis has been in the single digits in most contemporary series, with some reports describing prostheses surviving for 20 to 30 years and represents the gold standard against which contemporary material improvements will be measured over time. Beginning in 1997, the FDA approved a series of UHMWPE’s with elevated crosslinking for use in prosthetic joints. Their stated benefit is to dramatically decrease the generation of UHMWPE wear debris, confirmation of which finds support in wear simulator reports for hip and knee components. The commercial adaptation of these new UHMWPE’s has been aggressively marketed particular to acetabular components well in advance of now emerging short-term clinical reports. What follows is a descriptor of the clinical evolution of UHMWPE bearing surfaces: the good, the bad and the hopeful.

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Highly Crosslinked Polyethylenes: Hopes vs. Realities

The enduring success of the low-friction arthroplasty first advanced by Sir John Charnley as a solution for severe hip arthritic problems may be appreciated from the fact that in 2003 more than 800,000 hip and knee arthroplasties were performed in the United States. The prevalence of aseptic loosening attributed to ultra-high molecular weight polyethylene (UHMWPE) debris-induced osteolysis has been in the single digits in most contemporary series, with some reports describing prostheses surviving for 20 to 30 years and represents the gold standard against which contemporary material improvements will be measured over time. Beginning in 1997, the FDA approved a series of UHMWPE’s with elevated crosslinking for use in prosthetic joints. Their stated benefit is to dramatically decrease the generation of UHMWPE wear debris, confirmation of which finds support in wear simulator reports for hip and knee components. The commercial adaptation of these new UHMWPE’s has been aggressively marketed particular to acetabular components well in advance of now emerging short-term clinical reports. What follows is a descriptor of the clinical evolution of UHMWPE bearing surfaces: the good, the bad and the hopeful.

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Polymer Insert Stress in Total Knee Designs During High Flexion Activities: A Finite Element Study

The success of total knee arthroplasty has contributed to its widening application to a younger, more active patient population who daily regimen includes more demanding high flexion activities. Worldwide expansion to Middle Eastern and Asian populations, where the attainment of high knee joint flexion is often a cultural requirement, has been steadily increasing in recent years. This study reveals the contact areas and stresses that are associated with polymer insert abrasion in four total knee designs during the most highly loaded portions of three different high flexion activities, and suggests their efficacy in clinical use.

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Posted in Knee| Tagged , , , |

Tibial Plateau Abrasion in Mobile Bearing Knee Systems During Walking Gait III: A Finite Element Study

The abrasion observed in ultrahigh molecular weight polyethylene (UHMWPE) total knee arthroplasty component retrievals is the result of high cyclical loads, which act on the tibial plateau during daily ambulation. This dynamic process influences in vivo component longevity and is dependent on the magnitude and distribution of contact stresses on the tibial plateau. Mobile bearing knee systems offer increased component conformity over their fixed plateau counterparts and thus diminish the magnitudes of these contact stresses. This study reveals the contact areas and stresses that are associated with tibial insert abrasion in four mobile bearing knee designs during three highly loaded points in the walking gait cycle, and suggests their efficacy in clinical use.

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Scaphoid Fracture Repair: A Biomechanical Comparison of Contemporary Cancellous Bone Screws

Scaphoid fractures are common, but often challenging to treat. Clinic studies have demonstrated that both conservative treatment and internal fixation have successful long-term results with the latter providing earlier recovery of motion, decreased immobilization an dearly return to activity. Additionally, internal fixation is indicated as the preferred treatment for displaced or unstable scaphoid fractures, nonunions and late presenting fractures. Many surgeons have also advocated internal fixation for the treatment of acute nondisplaced fractures. Cancellous screw fixation of the scaphoid is one of the more popular and effective methods of treatment, as evidenced by the number of designs available to the orthopaedic surgeon. However, the clinical success of internal fixation is highly dependent upon the ability of the screw to obtain initial compression across the fracture site and its retention, under physiologic loading. This study compares the performance characteristics of seven, contemporary, scaphoid screw designs.

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New Polys for Old: Contribution or Caveat?

The enduring success of the low-friction arthroplasty first advanced by Sir John Charnley as a solution for severe hip arthritic problems may be appreciated from the fact that in 2001 more than 600,000 hip and knee arthroplasties were performed in the United States. The prevalence of aseptic loosening attributed to polyethylene debris-induced osteolysis has been in the single digits in most contemporary series, with some reports describing prostheses surviving for 20 to 30 years. Despite this obvious success, polyethylene debris is increasingly identified as the principle causative factor of bone loss and failure of TJA procedures. The conditions of occurrence, severity and clinical consequences suggest limitations on the in vivo integrity of contemporary arthroplasty designs. Material remedies inclusive of carbon reinforced polyethylene (Poly II), heat pressing and Hylamer with its variants have been advanced as alternatives with negative clinical results. Contemporary attempts to improve longevity (i.e., post-processing heat stabilization, peroxide additives and increased radiation with remelting) focus on the benefit of increased cross-linking while minimizing free radical creation. With increasing patient longevity and activity levels a search for the ultimate polymer is important. Variations on an already successful theme hope to lead to its optimization in this century. However, these “new polys” offer no direct clinical evidence to demonstrate their efficacy. It is known that reprocessing techniques will alter mechanical characteristics. Contemporary improvements in laboratory evaluative capabilities suggest significant reduction in component wear, but past experiences, in these regards, have not always predicted their clinical viability.

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Bone Graft Substitutes: Facts, Fictions & Applications

It is estimated that more than 500,000 bone-grafting procedures are performed annually in the United States, with approximately half of these procedures related to spine fusion. These numbers easily double on a global basis and indicate a shortage in the availability of musculoskeletal donor tissue traditionally used in these reconstructions. This reality has stimulated a proliferation of corporate interest in supplying what is seen as a growing market in bone-substitute materials. These graft alternatives have varying degrees of regulatory scrutiny and thus their true safety and effectiveness in patients may not be know prior to their use by orthopaedic surgeons. It is thus important to gain insight into this emerging class of bone-substitute alternatives. This handout provides an update of an emerging class of bone-graft substitutes, which have found application as osteoconductive fixation and structural media.

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